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| World Diabetes Day candle (Photo credit: martian cat) |
As a Type 1 Diabetic, controlling blood glucose levels is one of the reasons I workout. However, I have found that when I workout at a high intensity level, sometimes my levels go up. This usually happens, when my levels are already slightly elevated, prior to my workout. I was really perplexed by this, because my understanding, from talking with doctors and nutritionists, is that exercise increases insulin sensitivity, which should lower glucose levels. So, I have been researching this, and have come across the answer. It's really quite simple, actually.Like I said, exercise is generally great for lowering glucose because glucose is a rapidly metabolized form of energy, and when you exercise your body needs energy quickly. More often than not exercise will lower glucose, but there are certain circumstances under which blood glucose can actually increase from exercise.
Blood glucose can rise most commonly if your it is too high when you start exercising or if you exercise very strenuously. Both of these rises are caused by the same reason, not enough insulin to support the activity.
Reason #1 - Blood Glucose Level Too High to Start With
Blood Glucose being too high ALWAYS means that there is not enough insulin available to bring the it down to where it should be. Whether Type 1 or Type 2, insufficient effective insulin can cause highs and exacerbate highs during exercise.
When you exercise muscles need energy and the quickest form of energy is glucose. Muscles will increase their sensitivity temporarily during exercise so that the same amount of insulin can bring more glucose into muscle cells so that they can work. However, if there is insufficient insulin the muscles will not be able to receive enough glucose from the blood, even if there is more than enough glucose in the blood. As a result, the muscles send a signal that they need more energy, which the body responds to by releasing more glucose. However, since there is still not enough insulin, Blood Glucose rises and the muscles continue to send the signal for more energy. This is why if your Blood Glucose is high before exercising, exercise can drive your Blood Glucose up rather than down, the reason is that there is not enough insulin.
Reason #2 - Strenuous Exercise
The same reasons as above are the reasons why very strenuous exercise causes an increase rather than a decrease in Blood Glucose. During strenuous exercise the muscles send a signal for more energy, which the body responds to by releasing more glucose. Without sufficient insulin, very hard exercise and sometimes even new exercise (e.g. changing from body weight exercises to lifting heavy weights) will cause a rise in blood glucose.
At rest, the body uses about 60% of its energy as fat and 40% as glucose. The harder you work, the less fat is used and the more glucose is used until you reach a state of anaerobic activity (weight lifting, fast sprinting) which uses 100% glucose. It is counterintuitive, but the harder you exercise the more insulin your body needs to deal with the increased amount of glucose being released for energy. Often by exercising at a less strenuous pace, can cause BG to decrease with exercise again.
However, with all of that being said,
When exercise becomes anaerobic, glucose burns without the benefit of oxygen. And up to 17 times more of it is required. Because such large amounts of glucose are not available from the bloodstream and via the insulin transport mechanism, it is taken directly from the glycogen stored in the muscle (which is what causes muscle breakdown, if you don't have enough BCAAs in your system). Insulin is not needed for this. Insulin is used for the glucose to get into muscle cells so that it can be stored as glycogen. But it is not needed when glycogen derived glucose is burned in the muscle.
The hormone Glucagon is required for glycogen to be turned into glucose, and it is secreted into the bloodstream when adrenalin levels go up. Glucose made in this way from muscle glycogen can not get into the bloodstream and is burned by the muscle in which it was stored. But Glycogen stored in the liver is converted to glucose at the same time (because of the glucagon surging through the veins), and this glucose goes directly into the bloodstream. This is why anaerobic exercise causes blood glucose to go up.
Insulin requirements don't really come into it. Having said that, if insulin levels are low blood glucose is likely to rise sharply after anaerobic exercise. Lots of glucose is produced by the liver via gluconeogenesis so that glycogen stores can be rebuilt. And insulin is required for this.
As far as the pattern of energy usage is concerned, glucose is the primary source during the first 15-20 minutes of aerobic exercise. Initially, 70% of energy comes from carbohydrates. But it turns around after 20 minutes, after which fat becomes the primary source of energy. After you have been exercising for a while, 70% of the energy used comes from fat. But if the exercise becomes anaerobic, glycogen reserves (which are essentially glucose) will be drawn on. And the muscles will use as much glucose as they need.
Basically, if Blood Glucose is high before exercise, it is likely to rise significantly during.
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